A personal audio device, such as a wireless telephone, includes an adaptive noise canceling (ANC) circuit that adaptively generates an anti-noise signal from a reference microphone signal and injects the anti-noise signal into the speaker or other transducer output to cause cancellation of ambient audio sounds. An error microphone is also provided proximate the speaker to measure the ambient sounds and transducer output near the transducer, thus providing an indication of the effectiveness of the noise canceling. A processing circuit uses the reference and/or error microphone, optionally along with a microphone provided for capturing near-end speech, to determine whether the ANC circuit is incorrectly adapting or may incorrectly adapt to the instant acoustic environment and/or whether the anti-noise signal may be incorrect and/or disruptive and then take action in the processing circuit to prevent or remedy such conditions.
Examples of such Adaptive Noise Cancellation systems are disclosed in published U.S. Patent Application 2012/0140943, published on Jun. 7, 2012, and also in Published U.S. Patent Application 2012/0207317, published on Aug. 16, 2012, both of which are incorporated herein by reference. Both of these references are assigned to the same assignee as the present application and name at least one inventor in common and thus are not “Prior Art” to the present application, but are discussed herein to facilitate the understating of ANC circuits as applied in the field of use.
Referring now to FIG. 1, a wireless telephone 10 is illustrated in proximity to a human ear 5, or more specifically the pinna of a human ear. The pinna is the part of the human ear that extends from the head, and varies in shape and size between various individuals. As a result, the acoustical characteristics of a wireless telephone and the human ear will vary from person to person, based on the shape and size of their pinna 5. Moreover, how closely wireless telephone 10 is held to the pinna 5 will vary the acoustical characteristics and thus affect noise cancellation. For this reason as well as others, adaptive noise cancellation techniques are used to adaptively cancel background noise in a manner that is responsive to changes in the acoustical path between wireless phone 10 and pinna 5.
Wireless telephone 10 includes a transducer, such as speaker SPKR that reproduces distant speech received by wireless telephone 10, along with other local audio events such as ring tones, stored audio program material, injection of near-end speech (i.e., the speech of the user of wireless telephone 10) to provide a balanced conversational perception, and other audio that requires reproduction by wireless telephone 10, such as sources from web-pages or other network communications received by wireless telephone 10 and audio indications such as battery low and other system event notifications. A near-speech microphone NS is provided to capture near-end speech, which is transmitted from wireless telephone 10 to the other conversation participant(s).
Wireless telephone 10 includes adaptive noise canceling (ANC) circuits and features that inject an anti-noise signal into speaker SPKR to improve intelligibility of the distant speech and other audio reproduced by speaker SPKR. A reference microphone R is provided for measuring the ambient acoustic environment, and is positioned away from the typical position of a user's mouth, so that the near-end speech is minimized in the signal produced by reference microphone R. A third microphone, error microphone E, is provided in order to further improve the ANC operation by providing a measure of the ambient audio combined with the audio reproduced by speaker SPKR close to ear pinna 5, when wireless telephone 10 is in close proximity to ear pinna 5. Exemplary circuit 14 within wireless telephone 10 includes an audio CODEC integrated circuit 20 that receives the signals from reference microphone R, near speech microphone NS and error microphone E and interfaces with other integrated circuits such as an RF integrated circuit 12 containing the wireless telephone transceiver. CODEC 20 may incorporate ANC circuitry to provide adaptive noise cancellation.
In general, ANC techniques measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and also measures the same ambient acoustic events impinging on error microphone E. The ANC processing circuits of illustrated wireless telephone 10 adapt an anti-noise signal generated from the output of reference microphone R to have a characteristic that minimizes the amplitude of the ambient acoustic events at error microphone E.
Since acoustic path P(z) (also referred to as the Passive Forward Path) extends from reference microphone R to error microphone E, the ANC circuits are essentially estimating acoustic path P(z) combined with removing effects of an electro-acoustic path S(z) (also referred to as Secondary Path) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR including the coupling between speaker SPKR and error microphone E in the particular acoustic environment, which is affected by the proximity and structure of ear pinna 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, by the proximity and structure of ear pinna 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, and how firm the wireless telephone is pressed to ear pinna 5.
FIG. 2 is a block diagram illustrating the relationship between the elements of a type of ANC circuit known as Feed Forward ANC. The various types of ANC circuits (Feed-Forward, Feedback, and Hybrid) are described in more detail in the paper entitled On maximum achievable noise reduction in ANC systems, by A. A. Milani, G. Kannan, and I. M. S. Panahi, in Proc. ICASSP, 2010, pp. 349-352, published on March 2010 and incorporated herein by reference. The diagram of FIG. 2 is not an electrical block diagram, but rather illustrates the relationship of electrical, mechanical, and acoustical components in the overall system as shown in FIG. 1.
Input to the device is from reference microphone R, which outputs signal x(n) which represent the source of acoustic noise recorded by the reference microphone. The transfer function between the reference and error microphones is known as the Primary path P(z) or the passive forward path between error microphone E and the reference microphone R. Primary Path P(z) is represented in block 210. The noise signal after passing through P(z) is called d(n) which also represents the auto output received by error microphone E.
Secondary path S(z) is represented by block 230 and represents the transfer function of the electrical path, including the microphones E, R, and NS, digital circuitry (of FIG. 1), and canceling loudspeaker SPKR (of FIG. 1) plus the acoustical path between the loudspeaker SPKR (of FIG. 1) and the error microphone E. The input signal x(n) is fed to anti-noise filter 260 which has a transfer function W(z). The output y(n) from anti-noise filter 260 is then passed to adder 245, where it is added to a training signal (generally white noise) from Personal Entertainment System 290 (e.g., cellphone, pad device, or the like) and, after being inverted by inverter 255 (so as to subtract the resultant anti-noise signal) is input to secondary path transfer function 230. The output of this secondary path is added in adder 220 and the resultant signal e(n) is output to error microphone E via speaker SPKR (not shown).
SE(z) in block 280 represents an estimate of S(z). Due to the delay characteristics of the primary and secondary paths P(z), S(z), the feed-forward system of FIG. 2 may include an estimator to predict future noise and compensate for the delay characteristics in the overall system. Output signal e(n) is fed to adder 225 having an output that is inverted in inverter 235 and fed to least means square filter 250 which in turn generates a predicted S(z) filter value SE(z) in block 240. The output of block 240 in turn is fed into adder 225 in a feedback loop, so that this filter value is updated over time.
Predictive filter SE(z), that is shown as block 280, then accepts the input x(n) and uses the output through Least Means Squared filter 270 to create anti-noise filter value W(z) for anti-noise filter 260
The transfer function between the reference and error microphones is known as the Primary path P(z) or the passive forward path between error microphone E and the reference microphone R. The noise signal after passing through P(z) is called d(n).
Block 230 represents transfer function S(z) or the secondary path, which comprises the combined transfer functions of (a) a D/A converter, (b) a power amplifier, (c) speaker SPKR, (d) the air gap between speaker SPKR and error microphone E, (e) error microphone E itself, (f) an A/D converter, and (g) the physical structure of the audio device.
The ANC includes an adaptive filter (not shown) which receives reference microphone signal x(n), and under ideal circumstances, adapts its transfer function W(z) to be a ration of the primary path and secondary path (e.g., P(z)/S(z)) to generate the anti-noise signal. The coefficients of the adaptive filter 260 are controlled by a W(z) coefficient control block 260 that uses a correlation of two signals to determine the response of the adaptive filter, which generally minimizes, in a least-mean squares sense, those components of reference microphone signal x(n) that are present in error microphone signal.
The signals provided as inputs to LMS block 270 are the reference microphone signal x(n) as shaped by a copy of an estimate of the response of path S(z) provided by filter 280 and another signal provided from the output of a combiner 225 that includes the error microphone signal. By transforming reference microphone signal x(n) with a copy of the estimate of the response of path S(z),SE(z), and minimizing the portion of the error signal that correlates with components of reference microphone signal ref, adaptive filter 32 adapts to the desired response of P(z)/S(z).
One problem encountered in designing an adaptive noise cancellation system for a cellular telephone or other device is that the performance of an ANC system is very much dependent on the secondary path structure S(z). The secondary path contains the transfer functions of the D/A converter(s) and power amplifiers within integrated circuit 14, as well as the speaker, the air gap between the speaker and error microphone, the error microphone, A/D converter(s) within the integrated circuit 14, as well as the physical structure of the wireless telephone 10 itself.
Thus, in the prior art, a phone designer (or designer of other audio device) might place microphones and the speaker on the device based on aesthetic design criteria, or based on assumptions as to what would be a good location for a microphone or speaker. Only by building a testing model of the device could the designer evaluate the microphone and speaker placements. At that stage, it may be difficult to change the design if the microphone and speaker placements are found to be less than optimal. Moreover, testing each microphone and speaker combination and placement may be time consuming, particularly in terms of data acquisition and processing. Comparing different combinations of microphones and speakers and their placement, as well as phone case design and other secondary path variables may be difficult, as some combinations may provide superior performance in one frequency range, while others may provide better performance in other frequency ranges.
The inherent delay in the non-minimum phase S(z) is the major bottleneck which forces W(z) to be a predictor. This delay is mainly produced by the speaker transfer function and the air gap which corresponds to the relative placement of the speaker SPKR and the error microphone E. As a result, some of the zeros of S(z) fall outside the unit circle and make S(z) non-invertible. As transfer function W(z) is causal, if there is more delay, then the worse the performance of ANC system becomes. The physical structure and design of the audio system alter the transfer function S(z). There is no single metric that ANC designers and phone makers can use to evaluate the secondary path design (i.e., selection and placement of speaker and microphones, as well as the physical structure and design of the audio device).
Thus, it remains a requirement in the art to provide a metric and tool to evaluate secondary path design in an adaptive noise cancellation system, to allow designers to improve the design of such audio devices, and compare different designs more easily.